Const-eval more constants during MIR building

src/librustc_mir/hair/cx/expr.rs

@@ -514,17 +514,29 @@ fn convert_arm<'a, 'tcx: 'a>(cx: &mut Cx<'a, 'tcx>, arm: &'tcx hir::Arm) -> Arm<
 
 fn convert_path_expr<'a, 'tcx: 'a>(cx: &mut Cx<'a, 'tcx>, expr: &'tcx hir::Expr) -> ExprKind<'tcx> {
     let substs = cx.tcx.mk_substs(cx.tcx.node_id_item_substs(expr.id).substs);
-    match cx.tcx.def_map.borrow()[&expr.id].full_def() {
+    // Otherwise there may be def_map borrow conflicts
+    let def = { cx.tcx.def_map.borrow()[&expr.id].full_def() };
+    match def {
         def::DefVariant(_, def_id, false) |
         def::DefStruct(def_id) |
         def::DefFn(def_id, _) |
-        def::DefConst(def_id) |
-        def::DefMethod(def_id) |
-        def::DefAssociatedConst(def_id) =>
+        def::DefMethod(def_id) =>
             ExprKind::Literal {
                 literal: Literal::Item { def_id: def_id, substs: substs }
             },
 
+        def::DefConst(def_id) |
+        def::DefAssociatedConst(def_id) => {
+            if let Some(v) = cx.try_const_eval_literal(expr) {
+                ExprKind::Literal { literal: v }
+            } else {
+                ExprKind::Literal {
+                    literal: Literal::Item { def_id: def_id, substs: substs }
+                }
+            }
+        }
+
+
         def::DefStatic(node_id, _) =>
             ExprKind::StaticRef {
                 id: node_id,

src/librustc_mir/hair/cx/mod.rs

@@ -76,6 +76,13 @@ impl<'a,'tcx:'a> Cx<'a, 'tcx> {
         Literal::Value { value: const_eval::eval_const_expr(self.tcx, e) }
     }
 
+    pub fn try_const_eval_literal(&mut self, e: &hir::Expr) -> Option<Literal<'tcx>> {
+        let hint = const_eval::EvalHint::ExprTypeChecked;
+        const_eval::eval_const_expr_partial(self.tcx, e, hint, None)
+            .ok()
+            .map(|v| Literal::Value { value: v })
+    }
+
     pub fn partial_eq(&mut self, ty: Ty<'tcx>) -> ItemRef<'tcx> {
         let eq_def_id = self.tcx.lang_items.eq_trait().unwrap();
         self.cmp_method_ref(eq_def_id, "eq", ty)